Advances in electronic circuitry seem to be invariably linked to advances in circuit density of integrated circuits. With very large scale integration (VLSI) has come the need for packaging approaches which can provide high density, high speed interconnection between chips while minimizing crosstalk and controlling impedance, yet providing for effective heat removal and environmental protection of discrete components. There is no unique manner of achieving high density interconnections using multiple conductor layers on single boards, but recent years have seen an increased emphasis on, and utilization of, thin film multilayer structures using conventional metallic conductors and polymeric dielectrics.
Among the polymeric dielectrics utilized the class of polyimides (PI) by far are most frequently mentioned. Not only are they used as interlevel dielectrics, but also as passivants (protective coatings), as a filler in isolation trenches within semiconductor devices, and as soldering masks. For polymeric resins to be useful as, for example, a passivant, the material must be an excellent electrical insulator, must adhere well to the substrate, and must provide a barrier for transport of chemical species that could attack the underlying device. When used as an interlevel dielectric, the polymer must withstand 400.degree. C. without degradation of electrical, or mechanical properties. Additionally, the deposition, cure, and etch process must provide for reliable interconnection between the metal layers above and beneath the film. Stephen D. Senturia, Proceedings of the ACS Division of Polymer Materials: Science and Engineering, 55,385,386 (1986). To date polyimides are among the best available candidates to fill this demanding role.
Patterning polyimides can be performed in several ways including wet etching, plasma etching (or reactive ion etching, RIE) and direct photopatterning of photosensitive polyimides (PSPI). Wet etching can pattern only partially cured PI films and is limited to aspect ratios (thickness to feature width) of less than 1:1. RIE involves more steps and more expensive equipment than wet etching, but is capable of patterning high aspect ratio features with nearly vertical sidewalls in thick, fully cured PI film. Ronald J. Jensen, idem., 55,413,417 (1986). Because photopatterning requires fewer steps with significant cost and production advantages there is a great interest in its development, although significant problems remain to be overcome. Cf. Jensen, op. cit.
While the following review is not intended to be exhaustive, advances in and approaches to photopatterning polymeric dielectrics, chiefly as an interlevel dielectric, can be instructively chronicled through efforts of groups at DuPont, Hitachi, Toray, and IBM, and what follows is a brief sojourn through some relevant patent literature coming from these laboratories.
Initial processes employed a photoresist. In this method a coating of polyamide (polyimide precursor) was applied and the dried resin was overlayed with a photoresist, often baked on at moderate temperatures (under 100.degree. C.). The photoresist was irradiated through a masking pattern and subsequent development with an alkaline agent dissolved both the exposed portions of the photoresist and the underlying polyamide film. Exposed portions of the photoresist were then dissolved with, for example, acetone and the remaining polyamide acid film was cured at several hundred degrees centigrade.
Such conventional methods were perceived to have the disadvantages of affording only vias (or openings) between the layers of greater than 10 microns while obtaining good etch definition, and in U.S. Pat. No. 4,113,550 Saiki at Hitachi claimed to have found that etchants of hydrazines and alkaline diamines when used with semicured polyimide precursors in the general method above gave vias with a diameter down to about 3 microns with good etch definition in films about 1 micron thick. In 4,218,283 the method was extended to cured resins and then further extended in 4,436,583 by use of a special negative-type photoresist with reduced permeability to hydrazine-polyamine etchants.
Efforts at Toray were directed toward the elimination of the separate photoresist by providing a photosensitive composition which could be irradiated directly through a suitable mask and then developed with an appropriate solvent to provide the desired pattern. Some initial efforts used as photosensitive compositions a mixture of a polyimide precursor and a photopolymerizable olefin having an amino radical or quaternary ammonium group, such as cinnamic acid esters of disubstituted amino ethyl alcohol, and after irradiation through a mask the unmasked portions were developed by polar aprotic solvents such as N-methylpyrrolidone (NMP), dimethylformamide (DMF), N-methylacetamide (NMA), dimethylsulfoxide (DMSO) and hexamethylphosphoramide (HMP); U.S. Pat. No. 4,243,743. It needs to be emphasized that the polyimide precursors were not themselves photosensitive. The patentees mentioned that photoinitiators and photosensitizers also could be employed. Nonetheless, coatings with a thickness greater than 3 microns required strict control of development time to obtain a developed layer of relatively uniform thickness, and coatings under 3 microns were difficult to develop without degrading relief structures, evidently because of insufficient differential solubility of the photocrosslinked and non-crosslinked polymer precursor. In 4,547,455 this problem was addressed by using a new developer system of a mixture of the polar aprotic solvents previously used with methanol and the lower alkyl monoethers of diethylene glycol (Carbitols). This same problem appears to have been recognized by Kataoka et al. at Hitachi who in 4,565,767 described a light sensitive polymer composition of a polyamic ester, a special class of photosensitive bisazides, and a tertiary amine. An object of their work was greater differential solubility of photocrosslinked and unpolymerized resins. A further refinement was disclosed by Toray in 4,608,333 where the patentee teaches a photosensitive composition of polyimide precursor, a class of photodi- or photopolymerizable olefins as taught in '743, and certain aromatic or tertiary amines "chemically inactive to actinic radiation" where the mixture displayed improved radiation sensitivity, i.e., required less exposure time.
At DuPont the focus appeared to be on radiation sensitive polyimide precursors themselves. In U.S. Pat. No. 4,329,419 Goff teaches a polyamide ester resin (polyimide precursor) containing as photopolymerizable moieties polyfunctional acrylates in the ester portion of the resin, along with aromatic biimidazoles as photopolymerization initiators. The uncured resin film was irradiated through a mask and non-crosslinked resin was dissolved with, for example, polar aprotic solvents and monoethyl ethers of ethylene glycol. Exposure times of under 30 seconds were needed for film thickness of about 1.25 microns with subsequent development times of about 10 seconds. Compare also 4,410,612; 4,414,312; 4,416,973; 4,430,418; and 4,454,220.
This approach was echoed by Hitachi in 4,321,319 where the patentee taught a photosensitive polyamide acid resulting from reaction of a tetracarboxylic acid anhydride or tricarboxylic acid anhydride monohalide with a photosensitive diamine, the photosensitive group being, for example, an acrylate or cinnamate bound to the ring of an aromatic diamine. The photosensitive polymer also necessarily contains a photosensitizer. Kataoka et al. subsequently disclosed a variant where the light sensitivity of the polyamide acid arose from incorporation of an aromatic azide or sulfonyl azide, but still required a photosensitizer; U.S. Pat. No. 4,451,551.
The initial patents originating from IBM also appear to utilize a photoresist (U.S. Pat. Nos. 4,353,778 and 4,386,116) although direct etching with high energy ultraviolet light (40-400 nm) was disclosed in U.S. Pat. No. 4,508,749. The most relevant prior art for the purpose of our invention appears to be that of Araps et al., U.S. Pat. Nos. 4,568,601 and 4,656,050. The patentee teaches a method of irradiation and polymerization of dielectrics enabling the use of such polymeric dielectrics as photoresists in general. The invention facilitates in situ cure when the polymeric dielectrics are used to produce isolation films in multilayer devices, to fill isolation trenches within semiconductor structures, and for similar applications. The polymerizable oligomers are selected from the group consisting of poly N-substituted amic acids, the corresponding amic esters and isoimides, corresponding imides and mixtures thereof, where the polymerizable oligomer is end-capped with a vinyl or acetylenic moiety and can be coupled and crosslinked using radiation as the method of initiating reaction. The vinyl or acetylenic moieties responsible for the photosensitivity and photopolymerization of the oligomeric polyimide precursors are only at the termini of the oligomers and are bonded to terminal benzene rings attached to the imide nitrogen. It is noteworthy that the patentees used only electron beams irradiation as their method of curing and actually used only the acetylenic group for end-capping. When the unexposed portion was developed by 2-methoxyethyl ether image patterns as small as 0.5 microns wide could be obtained in a film somewhat greater than 2microns thick.
One striking feature which emerges from the foregoing review is the invariant use of polyimides as the photodefinable polymeric resin. However, such use of polyimide precursors and polyimides is accompanied by problems inherent in these materials themselves. Thus, an unacceptably large amount of volatiles is generated in the thermocycle necessary for curing of the polyimide precursor. These volatiles, which include water, carbon dioxide, nitrogen, and low molecular weight acrylates, lead to voids and cracks in the cured resin and to shrinkage in its preparation. Polyimides are hygroscopic which affects dielectric constant, and the changes in the dielectric constant with relative humidity often necessitate that the device be hermetically sealed. Polyimides also exhibit poor adhesion to themselves and to metals. A further defect with polyimides is that their coefficient of thermal expansion may be too high relative to other materials on the densely packaged electronic circuit.
In contrast, the photodefinable and photopolymerizable oligomers of our invention generate no volatiles either in the photochemical or thermal curing cycle. The cured resin exhibits a low coefficient of thermal expansion, especially along the z axis. The cured resin absorb a very low quantity of water, typically under 0.3% at 50% relative humidity, ensuring that the dielectric constant is relatively insensitive to humidity changes. In addition, the cured resins of this invention typically have a lower dielectric constant than polyimides, thereby permitting denser packaging than is the case with polyimide resins. Our cured resins also show a high glass transition temperature with good selfadhesion and good adhesion to metals. The photosensitive oligomeric resins of our invention are soluble in many converted solvents, in contrast to the need for using polar aprotic solvents for polyimides, which generally simplifies working with our resin and permits the use of many relatively low boiling solvents. The photosensitive resins of this invention also exhibit excellent planarization of the polymer, a particularly desirable property of interlevel dielectrics. Our materials photochemically crosslink throughout the thickness of the film, that is, there is no self-absorption of irradiation, with the result that the pattern shows no undercutting on development. In summary, and quite concisely, the materials of this invention provide photosensitive and photopolymerizable resins with excellent resolution and edge definition, superior differential solubility between the photocrosslinked and noncrosslinked oligomeric resin, do not need a photosensitizer or photoinitiator, and afford a cured resin which is superior in most of its properties to polyimides, the current industry standard.